Interworking between WLANs and 3G - Part 1: Interworking objectives & approaches

Based on the objectives and requirements discussed in the previous section, we present interworking schemes to connect WLANs and 3GPP networks [5–10].

Basically, interworking schemes can be categorized as mobile IP approach, gateway approach, and emulator approach. Mobile IP approach (called loose coupling approach), introduces mobile IP to two networks. Mobile IP mechanisms can be implemented in the mobile nodes and installed on the network devices of 3G and WLANs. This approach provides IP mobility for roaming between 3G and WLANs.

However, this approach requires installing mobile IP devices such as a home agent (HA) and a foreign agent (FA) in both networks, and terminal devices should also implement mobile IP features. Since the user device requires sending the registration back to its home network, packet delay and loss are also a problem for handoffs. Moreover, this approach suffers from the triangular routing between networks if mobile IP does not support route optimization (see Figure 22.1).

The gateway approach introduces a new logical node to connect two wireless networks. The new node is located between the two networks and acts as an internal device. It exchanges necessary information between the two networks, converts signals, and forwards the packets for the roaming users.

This approach aims to separate the operations of two networks, which implies the two networks are peer-to-peer networks and can handle their subscriber independently. With the two network operators having a roaming agreement, the logical node helps two networks offer intersystem roaming. The advantages of this approach are that the two networks can be operated independently; packets for roaming users go through the node without processing by mobile IP; and handoff delay and loss can be reduced (see Figure 22.2).

The emulator approach (called tight coupling approach), uses WLAN as an access stratum in a 3G network. This approach replaces 3G access stratum by WLAN layer one and layer two. A WLAN access point (AP) can be viewed as a 3G network controller or a serving GPRS support node (SGSN).

The benefit of this approach is that mobile IP is not required. All packet routing and forwarding are processed by a 3GPP core network. The packet loss and delay can be reduced significantly. However, this approach lacks flexibility since two networks are tightly coupled. The operators of two networks should be the same in order to exchange much information.

Another disadvantage of this approach is that the gateway GPRS support node (GGSN) will be the single point to the Internet. All packets have to go through the GGSN first. GGSN and the core network become the bottleneck (see Figure 22.3).